Method of firing explosive rivets



Aug. 24, 1943. L. A. BURROWS ET AL 2 METHOD OF FIRING EXPLOSIVE RIVETS Filed June 13, 1941 lFg ii LALBurrows MA. Cool:

[ 3 M gi d/ INVENTORS ATTORNEY Patented Aug. 24, 1943 2,327,763 METHOD OF FIRING EXPLOSIVE RIVETS Lawton A. Burrows and Melvin A. Cook, Woodbury, and Robert M. Girdler, signors to E. I. du Pont de Nemours Sewell, N. J., as-

& Company, Wilmington, Del., a corporation of Delawar 6 Application June 13, 1941, Serial No. 397,878

1 Claim.

This invention relates to a novel and improved method of firing the explosive charge in rivets of the type known as explosive rivets.

Explosive rivets have recently found many places of use, where, for various reasons, the ordinary solid rivets have been difficult to apply, as in cases where two metal plates were to be joined together but where one side of the lapping plates was inaccessible for work. An explosive rivet comprises ordinarily a headed metal pin of the length desired having a cavity extending into theshank either from the unheaded or the headed end. This cavity is adapted to hold an explosive charge and ordinarily, one capable of ignition at elevated temperatures merely by application of heat. The explosive charge should be located at least in part at that portion of the rivet shank which is positioned slightly beyond the far surface of the metal plate most removed from the head of the rivet. When the explosive rivet s located is fired, the walls of the rivet. in the vicinity of said charge are bulged out and the rivet thereby holds the metal plates tightly together,

While various methods have been employed for firing the explosive charges in said rivets, for example by percussion, by fuse, or by use of a high voltage static discharge the most generally used method has been by means of an electric heat-ing device Which' has been applied to the rivet head and by thermal conductance, has

raised the temperature of the entire rivet until the ignition temperature of the explosive charge has been attained, for example between 100* and 150 C. While such treatment has been sufficient to bring about development of the desired explosive force, some disadvantages have been encountered. Considerable time has been required, for example, for bringing about ignition in some cases where a number of rivets were to be heated and where an oxide coating on the rivet head decreased the rate of heat transfer to the metal surface. An additional disadvantage came from the fact that, with some rivets, it was necessary to overheat the head excessively in order to cause heat to flow to the vicinity of the explosive rapidly enough to detonate it successfully. This overheating of the head tended to promote corrosion. A further disadvantage came in the damage frequently caused to the silver tip of the electric heating device under the influence of the high temperature necessary, along with the pressure of contact. A method of effecting the firing .of the explosive charge that would be simple and vantages mentioned, would be an tribution to the art.

An object of the present invention is a method of firing explosive rivets by a novel a .d improved important conpractlcal, yet free from the disad-- method. A further object is such a method whereby a sufficient amount of heat is communicated to the explosive charge without the necessity of raising the temperature of the rivet head excessively. A still further object is a method of bringing such exposive charges in rivets to their ignition points by electrical heating wherein the metal rivet has no direct electrical connection with the source of power. Additional objects will be disclosed as the invention is described more at length hereinafter.

We have found that the foregoing objects are accomplished, and the disadvantages of the prior art overcome, when we introduce an explosive charge of relatively low ignition point into the cavity of a rivet and bring about the ignition of said rivet charge by means of heating due to induction by high frequency current. While this makes unnecessary the establishment of a direct electric contact with the rivet, we prefer to apply pressure on the rivet through physical contact and thus assure utilization of an increased amount of induced energy. The explosive charge is brought to ignition when the rivet metal adjacent to the explosive charge has attained the temperature corresponding to the explosion point of said charge. The term high frequency current will be readily understood. As is well-known, the ordinary commercial power and lighting circuits employ GO-cycle alternating current, where the surges back and forth take place 120 times per second. High frequency currents, however, as produced by the ordinary types of apparatus have frequencies upward of 1,000 oscillations per second, that is higher than 1,000 cycles or 1 kilocycle. Ordinarily, the currents applied will have frequencies greater than 15 kilocycles, and our preferred range will lie between and 10,000 kilocycles. Too low a frequency will be undesirable because of the cumbersome apparatus required while too high a frequency is undesirable because it favors surface heating. It will be understood therefore that when we speak of high frequency current, which functions'to fire the-rivet charge, we are not limiting ourselves to frequencies of a definite range, but only to those greatly in excess of the frequencies used in ordinary power lines, in other words, to currents which have the ability to heat efficiently by induction.

While the exact method of generating high frequency current is not an essential feature of this invention, one common way of accomplishing it is by use of an electron vacuum tube oscillator, whereby the desired frequency may be generated and then applied, for example, to the heating of the rivet by induction (eddy 'current) Another method for generating high frequency current is by use of the well-known spark-gap to the inductive heating of rivets.

oscillators. Such current may likewise be produced by means of electromechanical oscillators, various types of alternators, and the like. The source of electric power may be from direct current, from commercial 25- or 60-cycle alternating current, or other forms. It will be understood therefore that our invention covers the utilization of high frequency current, however produced and from whatever source, for the ignition of explosive charges in rivets, the heating of the rivet metal taking place by induction.

The method of carrying out our invention is illustrated specifically by reference to the appended drawing, in which Figure 1 shows schematically a means for generating high frequency current by use of a three-electrode vacuum tube (triode) and for applying said current Figure 2 shows a form of portable apparatus adapted for .use at the actual place of firing. Figure 3 illustrates a rivet with explosive charge in position, while Figure 4 shows the samerivet after the explosion.

With particular reference to Figure 1, I represents a plug for tapping a source of electric power, for example, (SO-cycle, l20-volt alternating current. 2 is a filament transformer between the power source and the triode 'tube 3. The plate transformer 4 supplies the proper plate voltage, with the radio frequency choke 5 interposed for preventing return of radio frequency current to the power supply. The tank circuit consisting of capacitance l and inductance 8 is excited by the plate of the vacuum tube through the blocking condenser 6. The coil 9 is shown closely adjacent to the rivet head, and high frequency current is applied from the coil to the rivet head at the desired moment by suitable control means. The rivet contains an explosive charge and this charge is exploded when the rivet becomes heated to the ignition point of said charge shortly after application of the high frequency current. While the firing coil and the rivet are shown directly adjacent to the source of the high frequency current in Figure 1, we preferably will employ a portable firing device and transmit the current to such device by suitable flexible leads.

Figure 2 is illustrates a portable firing device I I, to which the current from the high frequency course is brought by leads H. The coil 9 heats the rivet l0 and brings about the explosion of the charge.

Figure 3 shows the rivet I0, where it is to function in holding together the two aluminum sheets l3 and Id. The explosive charge is located in the cavity H. Figure 4 shows the rivet after explosion and its gripping'power on the metal sheets. The bulging effect is indicated at [2.

Using an apparatus assembly of this general nature, rivet charges were readily fired, Without undue heating of the oxide-coated heads thereof or of the rivet metal, in rapid succession.

Since the rivet charges are to be fired by the heating of the rivet metal to the ignition temperature of the explosive, it is important that the latter be one having a relati ely low ignition point and one that, upon ignition, picks up rapidly to high velocity of decomposition. Suitable charges of this nature have previously been disclosed and may comprise mixtures of lead azide and tetrazene; blends of nitromannite, tetrazene and aluminum; hexamethylene-triperoxydiamine; and many others. Explosive charges which can be ignited by the heat de veloped in the metal, as described above, may be designated as thermally-susceptible. Various forms of rivets have been proposed likewise, having the cavity for the explosive opening from the bottom of the shank or through the head, but such matters of rivet design are outside our invention. Preferably, we employ rivets with the cavity opening from the shank end.

The use of our invention offers a great advantage over the method previously employed. Whereas explosive rivets may be constructed of a number of metals such as iron, aluminum, magnesium, and others, or alloys thereof, aluminum an oxide coating, however, has very low thermal v conductivity. Since the prior art type of electric heater heats the rivets mainly by conduction, there is an excessive heating on the surface and loss of time in bringing the charge to its ignition temperature. Using our high-frequency current inductive heating method, the metallic body of the rivet is raised to the desired temperature of ignition without the necessity of overheating the non-conducting oxide coating. This is accomplished. because the rivet is heated rather uniformly throughout, due to the rapidly changing electromagnetic field at each point in the rivet produced by the high frequency current flowing in an adjacent wire of proper orientation. The consequent lower temperature of the surface of the rivet head is instrumental in preventing damage thereto, which would promote corrosion. The use of our high frequency inductive heating method makes possible the use of longer rivets than has been possible previously in the case of explosive rivets because of the improved heat transfer. In the heating methods of the prior art, also, it has been customary to employ electric heaters with silver tips for contacting the rivet heads. The high temperatures necessary for heating through the oxide coatings, together with the amount of pressure requisite, tend to cause excessive damage to the silver heating tips.

Our invention has been described at length in the foregoing, but it will he understood that many variations in procedures, compositions, and the like may be introduced without departure-from the spirit of the invention. We intend to be limited therefore only by the following patent claim.

We claim:

The method of firing charges in explosive metal rivets, which comprises effecting the heating of the charged rivet by means of induction applied at the head end from a high frequency electric current and thereby causing the ignition of said explosive charge, whereby said rivet head is not heated materially above the temperature necessary to cause the ignition of said charge, thus preserving the surface structure of the metal rivet;

LAWTON A. Buaaows. I MELVIN A. COOK. ROBERT M. GIRDLER. 

